Graphene represents a form of carbon in which the carbon atoms reside within a single atomically thin sheet or a few layered sheets (e.g., about 20 or less) of fused six-membered rings forming an extended planar lattice. In its various forms, graphene has garnered widespread interest for use in a number of applications, primarily due to its favorable combination of high electrical and thermal conductivity values, good in-plane mechanical strength, and unique optical and electronic properties. In many aspects, the properties of graphene parallel those of carbon nanotubes, since both nanomaterials are based upon an extended sp2-hybridized carbon framework. Other two-dimensional materials having a thickness of a few nanometers or less and an extended planar lattice are also of interest for various applications. In an embodiment, a two dimensional material has a thickness of 0.3 to 1.2 nm. In other embodiment, a two dimensional material has a thickness of 0.3 to 3 nm.
Because of its extended planar structure, graphene offers several features that are not shared with carbon nanotubes. Of particular interest to industry are large-area graphene films for applications such as, for example, special barrier layers, coatings, large area conductive elements (e.g., RF radiators or antennas), integrated circuits, transparent electrodes, solar cells, gas barriers, flexible electronics and the like. In addition, graphene films can be produced in bulk much more inexpensively at the present time than can carbon nanotubes.
Some envisioned applications for graphene and other two-dimensional materials are predicated upon forming a plurality of nanometer-scale holes in the planar structure of these nanomaterials. The process of forming holes in graphene and other two-dimensional materials will be referred to herein as “perforation,” and such nanomaterials will be referred to herein as being “perforated.” In a graphene sheet an interstitial aperture is formed by each six carbon atom ring structure in the sheet and this interstitial aperture is less than one nanometer across. In particular, this interstitial aperture is believed to be about 0.3 nanometers across its longest dimension (the center to center distance between carbon atoms being about 0.28 nm and the aperture being somewhat smaller than this distance). Perforation of sheets comprising two-dimensional network structures typically refers to formation of holes larger than the interstitial apertures in the network structure.
Perforation of graphene and other two-dimensional materials can modify the electrical properties of the material and its resistance to flow of fluid through the material. For example, the hole density of perforated graphene can be used to tune the electrical conductivity of this nanomaterial and in some instances can be used to adjust its band gap. Filtration applications are another area where perforated graphene and other perforated two-dimensional materials have generated considerable interest. Due to the atomic-level thinness of graphene and other two-dimensional materials, it can be possible to achieve high liquid throughput fluxes during filtration processes, even with holes being present that are only single-nanometer in size.
High performance, high selectivity filtration applications are dependent upon a sufficient number of holes of a desired size being present in a filtration membrane. Although a number of processes are known for perforating graphene and other two-dimensional materials, production of holes with a desired size range, a narrow size distribution, and a high hole density remains a challenge. At least one of these parameters is often lacking in conventional perforation processes.
Chemical techniques can be used to create holes in graphene and other two-dimensional materials. Exposure of graphene or another two-dimensional material to ozone or an atmospheric pressure plasma (e.g., an oxygen/argon or nitrogen/argon plasma) can effect perforation, but the holes are often lacking in terms of their density and size distribution. In many instances, it can be difficult to separately control hole nucleation and hole growth, so these processes often yield broad distributions of hole sizes. Further, many chemical perforation techniques produce holes that are at the extremes of 1) low hole density and small hole size, and 2) high hole density and large hole size. Neither of these extremes is particularly desirable for filtration applications. The first extreme is undesirable in terms of throughput flux, and the second extreme is undesirable for selectively excluding impurities that are smaller than the hole size.
Physical techniques can also be used to remove matter from the planar structure of two-dimensional materials in order to create holes. Hyperthermal ion beams tend to make pores in graphene and other two-dimensional materials that are too small for effective filtration to occur, primarily because the interaction of graphene and other two-dimensional materials with ions at hyperthermal velocities can be rather poor. The hyperthermal energy regime is defined as being intermediate between the thermal and low energy regimes. For example, a hyperthermal energy regime includes the energy range between 1 eV and 500 eV. Focused ion beams, in contrast, tend to make holes that are too few in number. Due to their very high energy flux, focused ion beams can also be exceedingly damaging to many substrates upon which the two-dimensional material is disposed. Because of their high energy requirements and small beam size, it is also not considered practical to utilize focused ion beams for perforating a large dimensional area.
Nanomaterials perforated with holes in a size range of about 0.3 nm to about 10 nm with a high hole density and narrow hole size distribution can be particularly difficult to prepare. Holes within this size range can be particularly effective for various filtration applications including, for example, reverse osmosis, molecular filtration, ultrafiltration and nanofiltration processes. As an example, holes in the size range 0.3 nm to 0.5 nm may be used for some gas separation processes. Holes in the size range 0.7 nm to 1.2 nm may be used for some desalination processes.
In view of the foregoing, scalable processes for perforating graphene and other two-dimensional materials in order to produce holes with a high hole density, narrow size distribution and a small hole size would be of considerable interest in the art. In particular, scalable processes to produce holes having a size, hole density, and size distribution suited for various filtration applications would be of considerable interest in the art. The present disclosure satisfies the foregoing needs and provides related advantages as well.